Methods for servicing subterranean wells

ABSTRACT

Process fluids comprising more than 1 wt % polyacrylamide and a non-metallic crosslinker may be used to control lost circulation in subterranean wells. The process fluid is placed into a subterranean lost-circulation zone and allowed to crosslink, thereby forming a gel barrier that limits further flow of process fluid into the zone. The non-metallic crosslinker may preferably comprise one or more polylactams. A pH-adjusting agent may also be incorporated into the process fluid.

BACKGROUND

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

This disclosure relates to methods for controlling lost circulation in subterranean wells, in particular, fluid compositions and methods for operations during which the fluid compositions are pumped into a wellbore, enter voids in the subterranean-well formation through which wellbore fluids escape, and form a seal that limits further egress of wellbore fluid from the wellbore.

During construction of a subterranean well, drilling and cementing operations are performed that involve circulating fluids in and out of the well. The fluids exert hydrostatic and pumping pressure against the subterranean rock formations, and may induce a condition known as lost circulation. Lost circulation is the total or partial loss of drilling fluids or cement slurries into highly permeable zones, cavernous formations and fractures or voids. Such openings may be naturally occurring or induced by pressure exerted during pumping operations. Lost circulation should not be confused with fluid loss, which is a filtration process wherein the liquid phase of a drilling fluid or cement slurry escapes into the formation, leaving the solid components behind.

Lost circulation can be an expensive and time-consuming problem. During drilling, this loss may vary from a gradual lowering of the mud level in the pits to a complete loss of returns. Lost circulation may also pose a safety hazard, leading to well-control problems and environmental incidents. During cementing, lost circulation may severely compromise the quality of the cement job, reducing annular coverage, leaving casing exposed to corrosive downhole fluids, and failing to provide adequate zonal isolation. Lost circulation may also be a problem encountered during well-completion and workover operations, potentially causing formation damage, lost reserves and even loss of the well.

Lost-circulation solutions may be classified into three principal categories: bridging agents, surface-mixed systems and downhole-mixed systems. Bridging agents, also known as lost-circulation materials (LCMs), are solids of various sizes and shapes (e.g., granular, lamellar, fibrous and mixtures thereof). They are generally chosen according to the size of the voids or cracks in the subterranean formation (if known) and, as fluid escapes into the formation, congregate and form a barrier that minimizes or stops further fluid flow. Surface-mixed systems are generally fluids composed of a hydraulic cement slurry or a polymer solution that enters voids in the subterranean formation, sets or thickens, and forms a seal that minimizes or stops further fluid flow. Downhole-mixed systems generally consist of two or more fluids that, upon making contact in the wellbore or the lost-circulation zone, form a viscous plug or a precipitate that seals the zone.

A thorough overview of LCMs, surface-mixed systems and downhole-mixed systems, including guidelines for choosing the appropriate solution for a given situation, is presented in the following reference: Daccord G, Craster B, Ladva H, Jones T G J and Manescu G: “Cement-Formation Interactions,” in Nelson E and Guillot D (eds.): Well Cementing-2^(nd) Edition, Houston: Schlumberger (2006): 202-219.

SUMMARY

The present disclosure provides means to seal voids and cracks in subterranean-formation rock, thereby minimizing or stopping fluid flow between the formation rock and the wellbore of a subterranean well.

In an aspect, embodiments relate to methods for controlling lost circulation in a subterranean well comprising: preparing a process fluid comprising more than 1 wt % polyacrylamide and a non-metallic crosslinker; placing the process fluid into a subterranean lost-circulation zone; and allowing the polyacrylamide to crosslink, thereby forming a gel barrier that limits further flow of process fluid into the zone.

In a further aspect, embodiments relate to methods for treating a subterranean well comprising preparing a process fluid comprising more than 1 wt % polyacrylamide and a non-metallic crosslinker; placing the process fluid into a subterranean lost-circulation zone; and allowing the polyacrylamide to crosslink, thereby forming a barrier that limits further flow of process fluid into the zone.

DETAILED DESCRIPTION

At the outset, it should be noted that in the development of any such actual embodiment, numerous implementation—specific decisions must be made to achieve the developer's specific goals, such as compliance with system related and business related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. In addition, the composition used/disclosed herein can also comprise some components other than those cited. In the summary of the disclosure and this detailed description, each numerical value should be read once as modified by the term “about” (unless already expressly so modified), and then read again as not so modified unless otherwise indicated in context. Also, in the summary of the disclosure and this detailed description, it should be understood that a concentration range listed or described as being useful, suitable, or the like, is intended that any and every concentration within the range, including the end points, is to be considered as having been stated. For example, “a range of from 1 to 10” is to be read as indicating each and every possible number along the continuum between about 1 and about 10. Thus, even if specific data points within the range, or even no data points within the range, are explicitly identified or refer to only a few specific, it is to be understood that inventors appreciate and understand that any and all data points within the range are to be considered to have been specified, and that inventors possessed knowledge of the entire range and all points within the range.

The following definitions are provided in order to aid those skilled in the art to understand the detailed description.

The term “treatment,” or “treating,” refers to any subterranean operation that uses a fluid in conjunction with a desired function and/or for a desired purpose. The term “treatment,” or “treating,” does not imply any particular action by the fluid.

As used herein, the term “polymer” or “oligomer” is used interchangeably unless otherwise specified, and both refer to homopolymers, copolymers, interpolymers, terpolymers, and the like. Likewise, a copolymer may refer to a polymer comprising at least two monomers, optionally with other monomers. When a polymer is referred to as comprising a monomer, the monomer is present in the polymer in the polymerized form of the monomer or in the derivative form of the monomer. However, for ease of reference the phrase comprising the (respective) monomer or the like is used as shorthand.

As used herein, the term “process fluid” refers to a pumpable fluid that may be circulated in a subterranean well. Such fluids may include (but not be limited to) drilling fluids, cement slurries, spacer fluids, pills, chemical washes, completion fluids, fracturing fluids, gravel-pack fluids and acidizing fluids. In this disclosure, pumpable fluids have a viscosity lower than 1000 mPa-s at a shear rate of 100 s⁻¹.

As used herein, the term “gel” refers to a solid or semi-solid, jelly-like composition that can have properties ranging from soft and weak to hard and tough. The term “gel” refers to a substantially dilute crosslinked system, which exhibits no flow when in the steady-state, which by weight is mostly liquid, yet behaves like a solid due to a three-dimensional crosslinked network within the liquid. It is the crosslinks within the fluid that give a gel its structure (hardness) and contribute to stickiness. Accordingly, gels are a dispersion of molecules of a liquid within a solid in which the solid is the continuous phase and the liquid is the discontinuous phase. A gel is considered to be present when the Elastic Modulus G′ is larger than the Viscous Modulus G,″ when measured using an oscillatory shear rheometer (such as a Bohlin CVO 50) at a frequency of 1 Hz and at 20° C. The measurement of these moduli is well known to one of minimal skill in the art, and is described in An Introduction to Rheology, by H. A. Barnes, J. F. Hutton, and K. Walters, Elsevier, Amsterdam (1997).

The term polyacrylamide refers to pure polyacrylamide homopolymer or copolymer with near zero amount of acrylate groups, a partially hydrolyzed polyacrylamide polymer or copolymer with a mixture of acrylate groups and acrylamide groups formed by hydrolysis and copolymers comprising acrylamide, acrylic acid, and/or other monomers.

This disclosure incorporates process fluids that preferably comprise more than 1 wt % polyacylamide crosslinked with a non-metallic crosslinker. The non-metallic crosslinkers do not include metals, but are instead organic molecules, oligomers, polymers, and/or the like. The polyacrylamide preferably has a weight average molecular weight higher than or equal to about 50,000 g/mol and lower than or equal to about 20 million g/mol, more preferably between about 500,000 g/mol and about 5 million g/mol. The polyacrylamide is preferably a partially hydrolyzed polyacrylamide having a degree of hydrolysis of from 0.01% up to less than or equal to about 40%, more preferably from 0.05% up to less than or equal to about 20%, and most preferably from 0.1% up to less than or equal to about 15%.

The non-metallic crosslinker may comprise a polylactam. Polylactams include (but are not limited to) any oligomer or polymer having pendent lactam (cyclic amide) functionality. Polylactams may be homopolymers, copolymers, block-copolymers, grafted polymers, or any combination thereof comprising from 3 to 20 carbon atoms in the lactam functional group pendent to the polymer backbone. Examples include (but are not limited to) polyalkyl-beta lactams, polyalkyl-gamma lactams, polyalkyl-delta lactams, polyalkyl-epsilon lactams, polyalkylene-beta lactams, polyalkylene-gamma lactams, polyalkylene-delta lactams, polyalkylene-epsilon lactams, and the like. Other examples of polylactams include polyalkylenepyrrolidones, polyalkylenecaprolactams, polymers comprising Vince lactam (2-azabicyclo[2.2.1]hept-5-en-3-one), decyl lactam, undecyl lactam, lauryl lactam, and the like. The alkyl or alkylene substituents in these polymers may include any polymerizable substituent having from 2 to about 20 carbon atoms, e.g., vinyl, allyl, piperylenyl, cyclopentadienyl, or the like. The non-metallic crosslinker may be polyvinylpyrrolidone, polyvinylcaprolactam, or a combination thereof. In the present disclosure, polyvinylpyrrolidone preferably has a weight average molecular weight higher than or equal to about 10,000 g/mol and less than or equal to about 2 million g/mol, more preferably higher than or equal to 50,000 g/mol and less than or equal to about 2 million g/mol.

Once crosslinking occurs, the process fluid may become a gel.

The gel preferably has a pH lower than or equal to about 3 or higher than or equal to about 9. Accordingly, the process fluid may further comprise a pH-adjusting agent. Such agents may be a base, an acid, a pH buffer, or any combination thereof. Such agents may include (but would not be limited to) sodium hydroxide, sodium carbonate, sulfuric acid, an organic acid, carbon dioxide or a combination thereof

In an aspect, embodiments relate to methods for controlling lost circulation in a subterranean well. A process fluid is prepared that comprises more than 1 wt % polyacrylamide and a non-metallic crosslinker. Further details concerning the process-fluid composition have been described earlier in the present disclosure. The process fluid is placed into a subterranean lost-circulation zone, whereupon the polyacrylamide therein is allowed to crosslink, thereby forming a gel barrier that limits further flow of process fluid into the zone.

To better control the location and the time at which the process fluid forms a gel, the polyacrylamide, the non-metallic crosslinker, the pH-adjusting agent or a combination thereof may be encapsulated. The encapsulation may be in the form of capsules with solid barriers, or in the form of emulsions wherein the polyacrylamide, non-metallic crosslinker, pH-adjusting agent or a combination thereof are in the internal phase. A combination of capsules and an emulsion may also be suitable. The capsule diameter may vary from about 1 μm and 5000 μm, preferably between about 10 μm and 2000 μm. The volume ratio between the coating and the material encapsulated within (coating:substrate) may vary between about 5:95 to 80:20, preferably between about 10:90 to 50:50. The capsule coating may comprise one or more dissolvable polymers selected from the list comprising polyvinylalcohol, partially saponified polyvinylalcohol, polyvinylpyrrolidone, methylcellulose, cellulose acetate, carboxymethylcelluose, hydroxyethylcellulose, polyethylene oxide, gelatin, dextrin, agar, pectin, polyvinylacetate, polyurea polymer and its derivatives such as polyurethane and copolymers of ethylene and vinyl acetate.

A trigger for releasing the encapsulated material may include (but would not be limited to) passage of the process fluid through the nozzles of a drill bit before the process fluid enters the lost circulation zone. The resulting shear may rupture the capsules or destabilize the emulsion. The trigger may be sufficient stress such as the passage through a restriction, e.g., a perforation or a drill bit. Without being bound by any theory, the inventors believe that the combination of shear and elongational flow experienced in these conditions are producing enough stress to break the shells made from a polymer inert for both the accelerator and the carrier fluid. Basically, the stress might first come from the turbulence experienced in the pumps of surface equipment and within the carrier fluid in itself; after that, the passage of the flow through a restriction creates first some sort of “Venturi effect” with an acceleration of the fluid which will have the effect of deforming the shells and then at the outlet of the restriction another deformation of the shells coming from fluid deceleration. Velocity increases and decreases are typically of the order of 50 to 100 times variation. Strain rates experienced in restriction are typically from 1 000 to one million reciprocal second, more specifically 10 000 to 200 000 reciprocal second. The inventors have noticed that even if the stress experienced during pumping and all along the transportation has an effect on the breakage of the shells, the stress and/or velocity difference which is obtained due to the flow through a restriction is of paramount importance. The stress is closely related to the pressure drop encompassed in each units of the well treatment (pumps, pipes, drill-bit). A higher pressure drop corresponds to a higher stress applied. Typically, the highest stress is observed when the fluid passes through the nozzles in a drill bit or a port of completion string downhole. By stress sufficient to break the shells, it is to be understood in the context of the present disclosure that said sufficient stress is produced by the passage through the nozzles of the drill bit or similar restriction to allow the accelerator to be released from the shells. Preferably, the pressure drop observed when passing through the nozzles is from about 150 to 5 000 psi (10 to 345 bar), more preferably from 300 to 5 000 psi (20 to 345 bar), most preferably from 300 to 1 000 psi (20 to 69 bar). As shown earlier, the stress may sometimes also be referred to as a velocity difference.

In a further aspect, embodiments relate to methods for treating a subterranean well. A process fluid is prepared that comprises more than 1 wt % polyacrylamide and a non-metallic crosslinker. Further details concerning the process-fluid composition have been described earlier in the present disclosure. The process fluid is placed into a subterranean lost-circulation zone, whereupon the polyacrylamide therein is allowed to crosslink, thereby forming a gel barrier that limits further flow of process fluid into the zone.

To better control the location and the time at which the process fluid forms a gel, the polyacrylamide, the non-metallic crosslinker, the pH-adjusting agent or a combination thereof may be encapsulated. The encapsulation may be in the form of capsules with solid barriers, or in the form of emulsions wherein the polyacrylamide, non-metallic crosslinker, pH-adjusting agent or a combination thereof are in the internal phase. A combination of capsules and an emulsion may also be suitable. The capsule diameter may vary from about 1 μm and 5000 μm, preferably between about 10 μm and 2000 μm. The volume ratio between the coating and the material encapsulated within (coating:substrate) may vary between about 5:95 to 80:20, preferably between about 10:90 to 50:50. The capsule coating may comprise one or more dissolvable polymers selected from the list comprising polyvinylalcohol, partially saponified polyvinylalcohol, polyvinylpyrrolidone, methylcellulose, cellulose acetate, carboxymethylcelluose, hydroxyethylcellulose, polyethylene oxide, gelatin, dextrin, agar, pectin, polyvinylacetate, polyurea polymer and its derivatives such as polyurethane and copolymers of ethylene and vinyl acetate.

For emulsions, the stabilizing surfactant preferably has a hydrophilic-lipophilic balance (HLB) value between about 8 and 18, and more preferably between about 12 and 16. A list of suitable surfactants may be found in the following publication: Ash M and Ash I: Handbook of Industrial Surfactants (4^(th) Edition), Synapse Information Resources, Endicott, New York (2005). Those skilled in the art will also recognize that Pickering emulsions, which employ solid particles to provide stability, may also be envisioned in the context of the present disclosure.

A trigger for releasing the encapsulated material may include (but would not be limited to) passage of the process fluid through the nozzles of a drill bit before the process fluid enters the lost circulation zone. The resulting shear may rupture the capsules or destabilize the emulsion. The trigger may be sufficient stress such as the passage through a restriction, e.g., a perforation or a drill bit. Without being bound by any theory, the inventors believe that the combination of shear and elongational flow experienced in these conditions are producing enough stress to break the shells made from a polymer inert for both the accelerator and the carrier fluid. Basically, the stress might first come from the turbulence experienced in the pumps of surface equipment and within the carrier fluid in itself; after that, the passage of the flow through a restriction creates first some sort of “Venturi effect” with an acceleration of the fluid which will have the effect of deforming the shells and then at the outlet of the restriction another deformation of the shells coming from fluid deceleration. Velocity increases and decreases are typically of the order of 50 to 100 times variation. Strain rates experienced in restriction are typically from 1 000 to one million reciprocal second, more specifically 10 000 to 200 000 reciprocal second. The inventors have noticed that even if the stress experienced during pumping and all along the transportation has an effect on the breakage of the shells, the stress and/or velocity difference which is obtained due to the flow through a restriction is of paramount importance. The stress is closely related to the pressure drop encompassed in each units of the well treatment (pumps, pipes, drill-bit). A higher pressure drop corresponds to a higher stress applied. Typically, the highest stress is observed when the fluid passes through the nozzles in a drill bit or a port of completion string downhole. By stress sufficient to break the shells, it is to be understood in the context of the present disclosure that said sufficient stress is produced by the passage through the nozzles of the drill bit or similar restriction to allow the accelerator to be released from the shells. Preferably, the pressure drop observed when passing through the nozzles is from about 150 to 5 000 psi (10 to 345 bar), more preferably from 300 to 5 000 psi (20 to 345 bar), most preferably from 300 to 1 000 psi (20 to 69 bar). As shown earlier, the stress may sometimes also be referred to as a velocity difference.

Those skilled in the art will recognize that the process fluids may comprise water, i.e., an aqueous gel, and/or an organic solvent. The organic solvent may be selected from the group consisting of diesel oil, kerosene, paraffinic oil, crude oil, LPG, toluene, xylene, ether, ester, mineral oil, biodiesel, vegetable oil, animal oil and mixtures thereof. Specific examples of suitable organic solvent include acetone, acetonitrile, benzene, 1-butanol, 2-butanol, 2-butanone, t-butyl alcohol, carbon tetrachloride, chlorobenzene, chloroform, cyclohexane, 1,2-dichloroethane, diethyl ether, diethylene glycol, diglyme (diethylene glycol dimethyl ether), 1,2-dimethoxy-ethane (glyme, DME), dimethylether, dibutylether, dimethyl-formamide (DMF), dimethyl sulfoxide (DMSO), dioxane, ethanol, ethyl acetate, ethylene glycol, glycerin, heptanes, Hexamethylphosphoramide (HMPA), Hexamethylphosphorous triamide (HMPT), hexane, methanol, methyl t-butyl ether (MTBE), methylene chloride, N-methyl-2-pyrrolidinone (NMP), nitromethane, pentane, Petroleum ether (ligroine), 1-propanol, 2-propanol, pyridine, tetrahydrofuran (THF), toluene, triethyl amine, o-xylene, m-xylene, p-xylene, combinations thereof, and/or the like.

Further solvents include aromatic petroleum cuts, terpenes, mono-, di- and tri-glycerides of saturated or unsaturated fatty acids including natural and synthetic triglycerides, aliphatic esters such as methyl esters of a mixture of acetic, succinic and glutaric acids, aliphatic ethers of glycols such as ethylene glycol monobutyl ether, minerals oils such as vaseline oil, chlorinated solvents like 1,1,1 -trichloroethane, perchloroethylene and methylene chloride, deodorized kerosene, solvent naphtha, paraffins (including linear paraffins), isoparaffins, olefins (especially linear olefins) and aliphatic or aromatic hydrocarbons (such as toluene). Terpenes are suitable, including d-limonene, 1-limonene, dipentene (also known as 1-methyl-4-(1-methylethenyl)-cyclohexene), myrcene, alpha-pinene, linalool and mixtures thereof

Further exemplary organic liquids include long chain alcohols (monoalcohols and glycols), esters, ketones (including diketones and polyketones), nitrites, amides, amines, cyclic ethers, linear and branched ethers, glycol ethers (such as ethylene glycol monobutyl ether), polyglycol ethers, pyrrolidones like N-(alkyl or cycloalkyl)-2-pyrrolidones, N-alkyl piperidones, N, N-dialkyl alkanolamides, N,N,N′,N′-tetra alkyl ureas, dialkylsulfoxides, pyridines, hexaalkylphosphoric triamides, 1,3-dimethyl-2-imidazolidinone, nitroalkanes, nitro-compounds of aromatic hydrocarbons, sulfolanes, butyrolactones, and alkylene or alkyl carbonates. These include polyalkylene glycols, polyalkylene glycol ethers like mono (alkyl or aryl) ethers of glycols, mono (alkyl or aryl) ethers of polyalkylene glycols and poly (alkyl and/or aryl) ethers of polyalkylene glycols, monoalkanoate esters of glycols, monoalkanoate esters of polyalkylene glycols, polyalkylene glycol esters like poly (alkyl and/or aryl) esters of polyalkylene glycols, dialkyl ethers of polyalkylene glycols, dialkanoate esters of polyalkylene glycols, N-(alkyl or cycloalkyl)-2-pyrrolidones, pyridine and alkylpyridines, diethylether, dimethoxyethane, methyl formate, ethyl formate, methyl propionate, acetonitrile, benzonitrile, dimethylformamide, N-methylpyrrolidone, ethylene carbonate, dimethyl carbonate, propylene carbonate, diethyl carbonate, ethylmethyl carbonate, and dibutyl carbonate, lactones, nitromethane, and nitrobenzene sulfones. The organic liquid may also be selected from the group consisting of tetrahydrofuran, dioxane, dioxolane, methyltetrahydrofuran, dimethylsulfone, tetramethylene sulfone and thiophen.

Those skilled in the art will also recognize that the process fluid may further comprise one or more viscosifiers. Some non-limiting examples of viscosifiers include (but are not limited to) hydratable gels (e.g., guars, poly-saccharides, xanthan, hydroxy-ethyl-cellulose, etc.), a crosslinked hydratable gel, a viscosified acid (e.g., gel-based), an emulsified acid (e.g., oil outer phase), an energized fluid (e.g., an N2 or CO2 based foam), viscoelastic surfactants (VES) and an oil-based fluid including a gelled, foamed, or otherwise viscosified oil. Additionally, the carrier fluid may be a brine and/or may include a brine.

The VES may be selected from the group consisting of cationic, anionic, zwitterionic, amphoteric, nonionic and combinations thereof. The viscoelastic surfactants, when used alone or in combination, are capable of forming micelles that form a structure in an aqueous environment that contribute to the increased viscosity of the fluid (also referred to as “viscosifying micelles”). These fluids are normally prepared by mixing in appropriate amounts of VES suitable to achieve the desired viscosity. The viscosity of VES fluids may be attributed to the three dimensional structure formed by the components in the fluids. When the concentration of surfactants in a viscoelastic fluid significantly exceeds a critical concentration, and in most cases in the presence of an electrolyte, surfactant molecules aggregate into species such as micelles, which can interact to form a network exhibiting viscous and elastic behavior.

A zwitterionic surfactant of the family of betaine may be used. Exemplary cationic viscoelastic surfactants include amine salts and quaternary ammonium salts. Exemplary amphoteric viscoelastic surfactant systems include for example amine oxides and amidoamine oxides. Mixtures of zwitterionic surfactants and amphoteric surfactants are suitable. An example is a mixture of about 13% isopropanol, about 5% 1-butanol, about 15% ethylene glycol monobutyl ether, about 4% sodium chloride, about 30% water, about 30% cocoamidopropyl betaine, and about 2% cocoamidopropylamine oxide. Suitable anionic surfactants include alkyl sarcosinates.

The process fluid may further comprise a degradable material. The degradable material may include (but would not be limited to) at least one of a lactide, a glycolide, an aliphatic polyester, a poly (lactide), a poly (glycolide), a poly (ε-caprolactone), a poly (orthoester), a poly (hydroxybutyrate), an aliphatic polycarbonate, a poly (phosphazene), and a poly (anhydride). The degradable material may also include a poly (saccharide), dextran, cellulose, chitin, chitosan, a protein, a poly (amino acid), a poly (ethylene oxide), and a copolymer including poly (lactic acid) and poly (glycolic acid). The degradable material may include a copolymer including a first moiety which includes at least one functional group from a hydroxyl group, a carboxylic acid group, and a hydrocarboxylic acid group, the copolymer further including a second moiety comprising at least one of glycolic acid and lactic acid. Such degradable materials may be employed to trigger a reduction in gel viscosity after a given time, thus making the gel reversible. Reversible gels may have utility for hydrocarbon producing zones that are also lost circulation zones. Having the ability to reverse the gels may be useful for reestablishing zonal productivity.

The process fluid may optionally further comprise additional additives, including, but not limited to, acids, fluid loss control additives, corrosion inhibitors, scale inhibitors, catalysts, clay control agents, biocides, friction reducers, combinations thereof and the like. 

1. A method for controlling lost circulation in a subterranean well, comprising: i. preparing a process fluid comprising more than 1 wt % polyacrylamide and a non-metallic crosslinker; ii. placing the process fluid into a subterranean lost-circulation zone; and iii. allowing the polyacrylamide to crosslink, thereby forming a gel barrier that limits further flow of process fluid into the zone.
 2. The method of claim 1, wherein the non-metallic crosslinker comprises a polylactam.
 3. The method of claim 1, wherein the non-metallic crosslinker comprises polyvinylpyrrolidone, polyvinylcaprolactam or a combination thereof.
 4. The method of claim 3, wherein the polyvinylpyrrolidone has a weight average molecular weight of greater than or equal to about 50,000 g/mol and less than or equal to about 2 million g/mol.
 5. The method of claim 1, wherein the non-metallic crosslinker has a weight average molecular weight greater than or equal to about 10,000 g/mol and less than or equal to about 2 million g/mol.
 6. The method of claim 1, wherein the polyacrylamide has a degree of hydrolysis less than or equal to about 40%.
 7. The method of claim 1, wherein the process fluid further comprises a pH-adjusting agent.
 8. The method of claim 7, wherein the polyacrylamide, non-metallic crosslinker, pH-adjusting agent or a combination thereof are encapsulated within a capsule or an emulsion or both.
 9. The method of claim 8, wherein the process fluid passes through a drill bit before entering the lost-circulation zone.
 10. The method of claim 9, wherein the encapsulated polyacrylamide, non-metallic crosslinker, pH-adjusting agent or a combination thereof are released upon passage through the drill bit, and the process-fluid pH is lower than or equal to about 3, or higher than or equal to about
 9. 11. A method for treating a subterranean well, comprising: i. preparing a process fluid comprising more than 1 wt % polyacrylamide and a non-metallic crosslinker; ii. placing the process fluid into a subterranean lost-circulation zone; and iii. allowing the polyacrylamide to crosslink, thereby forming a barrier that limits further flow of process fluid into the zone.
 12. The method of claim 1, wherein the non-metallic crosslinker comprises a polylactam.
 13. The method of claim 1, wherein the non-metallic crosslinker comprises polyvinylpyrrolidone, polyvinylcaprolactam or a combination thereof.
 14. The method of claim 3, wherein the polyvinylpyrrolidone has a weight average molecular weight of greater than or equal to about 50,000 g/mol and less than or equal to about 2 million g/mol.
 15. The method of claim 1, wherein the non-metallic crosslinker has a weight average molecular weight greater than or equal to about 10,000 g/mol and less than or equal to about 2 million g/mol.
 16. The method of claim 1, wherein the polyacrylamide has a degree of hydrolysis less than or equal to about 40%.
 17. The method of claim 1, wherein the process fluid further comprises a pH-adjusting agent.
 18. The method of claim 17, wherein the polyacrylamide, non-metallic crosslinker, pH-adjusting agent or a combination thereof are encapsulated within a capsule or an emulsion or both.
 19. The method of claim 18, wherein the process fluid passes through a drill bit before entering the lost-circulation zone.
 20. The method of claim 19, wherein the encapsulated polyacrylamide, non-metallic crosslinker, pH-adjusting agent or a combination thereof are released upon passage through the drill bit, and the process-fluid pH is lower than or equal to about 3, or higher than or equal to about
 9. 